1. Field of the Invention
The present invention relates to an exhaust gas purifying device to be applied to an exhaust gas purifying system for internal combustion engines such as diesel engines. The exhaust gas purifying system is generally comprised of an exhaust gas pipe, a urea water adding valve, an unit having selective catalytic reduction (SCR) capable of selectively purifying nitrogen oxide (NOx) contained in an exhaust gas emitted from the internal combustion engine by using ammonia as a reductant (or a reducing agent), and one or more NOx sensors.
2. Description of the Related Art
There have been developed selective catalytic reduction (SCR) systems, and some of them have been put in practice. Such a SCR system reduces nitrogen oxide (NOx) contained in an exhaust gas emitted from an internal combustion engine (in particular, a diesel engine) mounted to vehicles with a high purifying rate, etc.
In general, such a SCR system is comprised of an exhaust gas pipe, an unit having an NOx catalyst of a selective reduction type, and a valve through which urea water (as reductant solution) is supplied into the exhaust gas which flows in the exhaust gas pipe. The exhaust gas pipe is communicated with a diesel engine. The urea water adding valve is placed at an upstream side of the unit having NOx catalyst of a selective reduction type on the exhaust gas pipe.
when urea water is supplied into the exhaust gas in the exhaust gas pipe through the valve, the urea water is hydrolyzed by the thermal energy of the exhaust gas. The hydrolysis of the urea water generates ammonia (NH3). The generated ammonia and NOx contained in the exhaust gas are reacted together. This selectively reduces NOx contained in the exhaust gas, and the exhaust gas is thereby purified.
In order to increase the NOx purifying rate to purify NOx contained in the exhaust gas by using the NOx catalyst, and to suppress occurrence of ammonia slip in which surplus ammonia generated in NOx reduction reaction remains and is discharged through the exhaust gas pipe, it is necessary to supply an optimum amount of ammonia to the amount of NOx catalyst. In order to achieve this control, the conventional SCR system is equipped with one or more NOx sensors, at least one placed at a downstream side of the unit containing NOx catalyst. Based on a detection signal of the NOx sensor, it is possible to control the opening rate of the valve in order to adjust the amount of urea water, and to diagnose occurrence of an abnormal state in the SCR system.
By the way, there is a special NOx sensor in various types of NOx sensors which are currently available in the market, which is capable of detecting ammonia in addition to NOx. However, it is difficult to specify whether a detection signal of the NOx sensor is generated by the presence of NOx or ammonia. Even if there is a high probability of abnormal state when the NOx sensor placed at a downstream side outputs a detection signal of a large value, it is impossible to determine whether such a large detection signal from the NOx sensor is caused by the occurrence of ammonia slip or the presence of NOx contained in the exhaust gas. Therefore the above type of a NOx sensor cannot specify the cause of generating such abnormal condition, namely, which is caused by the occurrence of ammonia slip or the presence of NOx.
In order to solve such a conventional problem, there has been proposed a conventional technique to detect an occurrence of ammonia slip based on a detection signal of a NOx sensor. (For example, such a conventional technique is disclosed in Japanese patent laid open publication No. JP 2003-293743).
The conventional technique disclosed in JP 2003-293743 increases an adding amount of urea water when an actual NOx purifying rate is lower than a target NOx purifying rate, and judges the occurrence of ammonia slip when the actual NOx purifying rate is not increased
Further, when judging the occurrence of ammonia slip, the conventional technique decreases an ammonia adsorption amount on the NOx catalyst in order to prevent the ammonia slip. The conventional technique then judges the SCR system to be in an abnormal state when the NOx purifying rate is not increased according to decreasing the ammonia adsorption amount of NOx catalyst.
When the actual NOx purifying rate is lower than the target NOx purifying rate, it can be considered that ammonia is discharged into the downstream side of the unit of NOx catalyst due to an excess supply of urea water in addition to when the exhaust gas containing NOx is discharged into the downstream side of the unit of NOx catalyst by lacking an adequate amount of urea water. Accordingly, when the amount of urea water is increased when the actual NOx purifying rate is lower than the target NOx purifying rate, this promotes discharging ammonia into the exhaust gas in the exhaust gas pipe. That is, there is a probability of increasing the ammonia slip when the added amount of urea water is forcedly changed without determining whether the increasing of the output value of the NOx sensor placed at the downstream side of the unit having NOx catalyst is caused by ammonia slip or the presence of NOx contained in the exhaust gas.
Accordingly, it is preferable to more decrease the number of processes to execute the abnormality diagnosis while forcedly changing the adding amount of urea water.
In the point of view to improve the accuracy of the abnormality diagnosis, it is preferred to execute the abnormality diagnosis based on the detection signal of the NOx sensor under the condition in which the detection signal of the NOx sensor is free from various types of fluctuations.